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1. Field of the Invention
This invention generally relates to systems for determining the position and orientation of an object which may deform in shape over time and which use the detection of energy emitted from markers placed on the object.
2. Description of the Related Art
As is known in the art, passive systems exist which rely on the markers being illuminated with energy that reflects off the markers and is detected by the sensor system. Active systems also exist in which the markers are individual sources of energy. In both cases the energy is focused onto spaced sensors, such that the position of an energised marker is identified by the sub set of adjacent sensor points that are recording an energy level above a given threshold.
By identifying which adjacent sensors are detecting energy above the threshold, the associated computing devices can estimate the position of the marker emitting the energy in a given plane in space since the focusing function relates a point in space to a sensor on the sensor system. To achieve a high resolution measurement from such systems a very large number of sensors need to be positioned adjacent to each other as each sensor relates to a point in space. Having a large number of sensors leads to a degradation in the capture rate as the signal levels must be digitised for a large number of sensors. By using three displaced sensor sets, the position of the marker can be calculated to a certain level of accuracy in 3 dimensional space.
In such systems energy from the marker is directly focused onto the sensors so that only a small number of sensors detect energy over the given threshold. Such systems do not measure the distribution of energy levels across a large percentage of the total number of sensors and do not calculate the position of the marker based on an energy distribution function for which a maximum value occurs for a calculated marker position.
In passive systems all illuminated markers are energised and detected simultaneously.
Therefore the computing device needs firstly to identify which sub set of adjacent sensors detecting energy above the given threshold correspond to which marker. Secondly it must track each marker from one sample to the next and attempt to distinguish each marker at all times. This results in the possibility of errors where marker assignments are lost and requires intensive processing methods.
Active systems may illuminate all markers at the same time or they can cycle the illumination of each marker to aid the computing system distinguish individual markers. If all markers illuminate at the same time, the computing device must be able to identify the correspondence of each marker and each energy detection and it must then track each marker in a similar way to the passive system.
In an active system that illuminates each marker individually, the computing device can immediately make the correspondence of marker energy emission and detection since the cycle time will be known. As each energy emission is recorded separately, no tracking is required and the position is simply calculated for each illumination. In one such system the sensor set is a multiple charge coupled device (CCD) onto which the energy is focused. To detect in 3D space at least three CCD detectors are used. In order to achieve high measurement resolution the CCD must have a large number of detecting sensors since the focusing function relates a point in space to each sensor point on the CCD. In order to achieve very high sample rates for a large number of markers the CCD must be driven at very high rates, well above the needs of conventional CCD devices. High resolution CCD devices capable of working at high data capture rates are expensive. In addition systems that use CCD devices have a measurement non-linearity dependent on the ability of the lens component to accurately focus the marker points linearly across the CCD sensor surface and not introduce any aberration.
As the CCD is moved further away from the markers on the object of interest, the measurement scaling changes since the focused image of the markers on the sensor system changes in size. Due to lens aberration and changes in measurement scaling such systems require a calibration phase in which an object of known dimensions is introduced for sampling. This calibration task is inconvenient, often needs experienced personnel to perform it and is considered a negative aspect from the point of view of end user.
In accordance with the present invention, a system for determining the position, orientation and deformation in 3 dimensional space of a moving object in real-time is provided having a plurality of activatable markers which are mounted onto parts of the object for which a position value needs to be recorded, a drive unit which drives the activatable markers in a defined sequence, a sensor section comprising a plurality of sensors remote from the markers and suitably arranged such that the energy falling on the sensor is dependent on the relative position of the energised marker and the sensor, a data capture unit which digitises the signals sensed by each sensor and a control unit which processes the signals received from the data capture unit.
The object may move in space and may deform in shape and the markers are mounted so as to move with the object and follow the shape deformation.
Each marker is activated individually for a period of time by the drive unit in a sequence known to the data capture unit and the control unit. While each marker is illuminated the energy from the marker is detected by all sensors. The sensors are arranged such that the energy distribution sensed by the plurality of markers for a single energised marker is a function of the position of the marker. The digitised energy levels are transmitted to the control unit at high speed and the information is processed to determine the position of the marker. The control unit calculates the position of the marker based on an energy distribution function for which a maximum value occurs for a calculated marker position.
By using this approach relatively few sensors are needed to determine marker position. This results in low digitisation and data collection overheads and therefore faster sample rates than if CCD devices were used. In addition by using a much lower number of sensor components significant cost reductions are achievable.
It is important to stress that the amplitude of the energy signal is not used to determine the position of the marker rather it is the energy distribution pattern over a number of sensors. For example the distance from the marker to a sensor is not calculated using the energy amplitude detected per se; in which case the emitter and sensor would need to be finely calibrated.
Since accurate signal strength values are unnecessary calibration of the energy emission and detection components is unnecessary.
Since the system relies on the way energy is distributed over the sensors, there is no need to calibrate the system for measurement scaling.
Since only one marker is activated at one time during a single cycle, the sensor section can individually determine the position of each marker as each marker is separately illuminated thereby making marker tracking unnecessary.
To reduce significantly the effect of external ambient energy radiation being superimposed upon the energy signal emitted by each marker the drive unit can split each specific marker illumination period into two parts. In the first part the marker is fully illuminated and the signal level detected is digitised. During a second part the marker is not illuminated and instead the data capture unit samples the ambient energy signal for each sensor and digitises it. Since the time interval between the two samples is very small the ambient energy level can be assumed to equal the subsequent ambient energy signal recorded and since the effect of ambient energy on marker energy detection can be considered to follow a simple superimposition rule, the final signal level attributed to the marker is the illuminated signal level less the ambient signal level. This subtraction can be performed by the data capture unit and the result can be transmitted to the control unit.
To deal with possible saturation of sensors due to the marker being too close to a group of sensors the drive unit can be designed to drive the marker with maximum illumination followed by a period of illumination at 50% of maximum, followed by the zero illumination level for the purposes of ambient energy detection. In cases where the maximum illumination level results in a saturation of the sensors, the data capture unit can globally choose to use the energy levels recorded during the time at which the drive unit illuminated the marker at 50% of maximum drive. In this way the system can automatically adapt to saturation of the sensors which may occur if the marker is positioned very close to the sensors in which case the signal values detected during the period of lower energy emission are used for the calculations.
In a preferred embodiment of the invention, the system includes: a plurality of infra red emitting markers that can be mounted to points of interest on the object; a drive unit which sequences the activation of the markers according to a synchronisation signal derived from the control unit; a sensor section, preferably a linear array of infra red sensors where each sensor is set approximately 75 mm apart and placed behind a linear collimator slot orthogonal to the linear array axis; a data capture section comprising a set of microprocessors which read the sensor levels and send the data to a control unit; a control unit comprising a processor to calculate the position of each marker and a data reception part for reception of the sensor level data from the data capture unit.
The sensors are provided with a collimator through which the energy may pass only up to a limited angle of incidence after which the sensor detects no energy. When the marker is directly positioned above a sensor the full energy of the marker is detected however as the marker moves away from the perpendicular axis adjoining the sensor, the energy level detected reduces as the collimator begins to attenuate the energy emitted by the marker.
The control unit activates the drive unit to begin the illumination of markers in a sequence known to the control unit. The sensor section detects the emitted energy and the data capture section digitises the levels and transmits them to the control unit. On reception the control unit analyses the data from a single sensor axis and calculates the most likely point along the length of the sensor axis which corresponds to the nearest point between the axis and the marker.
The control unit may receive data from several sensor axes, at least three, and will compute the 3D position of the marker in 3D space.
The control unit may display the information, store it or transmit it to another computing device for further use.